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Stellar Birth and Life Help (page 2)

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By — McGraw-Hill Professional
Updated on Sep 17, 2011

Life Summaries

Massive stars lead much different lives than small ones. Our Sun seems to be slightly “on the good side” of the dividing line between stable red and orange dwarfs and unstable blue and white giants.

Eventually, most of the hydrogen in the center of a star gets converted to helium by the fusion process. For nuclear reactions to continue, the core temperature must rise. As the star “runs out of gas,” it starts to cool, and gravitation overcomes the outward pressure caused by internal heating. This contraction causes the temperature to rise again, and this time it goes even higher than it did when the star was born. Eventually, it gets so hot that helium atoms begin to fuse, forming carbon atoms plus energy. Our Sun has not yet reached this stage.

Once the helium has been used up, the star contracts again, becoming hotter still; carbon atoms begin to fuse, forming heavier atoms. In this way, it is believed, all the 92 elements up to and including uranium were formed in the interiors of stars long ago. This includes all those atoms in our planet: all the iron ore, all the silicon, all the gold, and everything else. It even includes the atoms in your body, with the exception of hydrogen that forms part of the water that keeps you alive. But how did all these heavier elements get here from the centers of stars? The answer lies in the fate of the most massive stars: They are doomed to blow up.

Large stars have attracted much attention from astronomers. When stars age, they sometimes swell to extreme size, and their surface temperatures cool. They become red giants. This is expected to happen to our Sun in a few billion years. After the red-giant phase, the Sun will contract and gradually fade away. However, more massive stars undergo a sudden and violent death. For a few days they can become as bright as all the rest of the stars in the galaxy combined. Once in a while we see such a supernova from Earth. When a big star explodes, it hurls much of its matter into interstellar space, where it cools and becomes the stuff from which future star systems can form. Some of these systems are believed to develop into star-and-planet families similar to our Solar System. This explains how the heavy elements got here. If this theory is correct, we all owe our existence to one or more supernovae that took place billions of years ago.

Practice problems of this concept can be found at: Stars and Nebulae Practice Problems

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